FIG. 1 shows a cellular system 100. The cellular system 100 coverage area may be divided into separate areas defined by cells 101-107. Each mobile station 120-122 may operate within the coverage area of cellular system 100. Each cell 101-107 is controlled by a radio frequency transmitting and receiving base station 111-117 that allows base station-to-mobile and reverse-link (mobile station to base station) channels to be established. The reverse-link and forward-link channels provide paths for communication between mobile stations and an associated base station 111-117. Cellular system 100 may be implemented according to one of the many cellular system standards currently in use or proposed for use. For example, system 100 may be implemented in accordance with the Telecommunications Industry Association standard (TIA/EIA) IS-95 Mobile Station-Base Station Compatibility Standards for Dual-Mode Wideband Spread Spectrum Cellular System (“IS-95”) or in accordance with the Japanese Association of Radio Industries and Businesses (ARIB) standard Specification of Mobile Station for 3G Mobile Station System Version 1.0 (WCDMA) or other third-generation (3G) standards, such as the European multi-carrier CDMA (MC-CDMA) or TIA/EIA cdma2000 First Generation Standard (cdma-2000—1x).
In cellular system 100, a number of forward-link CDMA channels can share the same time-frequency space. Similarly, a number of reverse-link channels can share another time-frequency space. To enable this time-frequency sharing, CDMA channels are separately defined by performing an orthogonal conversion and spreading of information to be transmitted on the channel using Walsh codes. Walsh codes can be used to uniquely spread channel information in the same time-frequency space used by other channels using different Walsh codes. At a receiver, the unique Walsh code assigned to a particular channel is used to despread a received signal and to discriminate the desired channel from others occupying the same time-frequency space. The technique is also referred to as “spread spectrum” communication.
To receive and despread CDMA channels that are modulated in a particular radio frequency (RF) band (i.e., a particular frequency space), a mobile station down-converts the carrier frequency of a received RF band and provides the down-converted signal to one or more fingers of a RAKE receiver (also referred to as “finger” receiver). A RAKE receiver contains despreading correlators that can despread a particular channel using the Walsh code associated with that channel. A RAKE receiver may employ multiple fingers to improve multipath signal reception. Different dominant multipath components of a multipath signal can be despread by different RAKE receiver fingers. The RAKE receiver may then combine the despread signals from the different multipath components to maximize the despread energy. The RAKE receiver may combine the despread signals based on an estimate of phase and amplitude of the despread components.
In a second-generation cellular phone system, such as an IS-95 CDMA system, digitized information is transmitted from a base station to multiple mobile stations using a single 1.23 MHz RF band that contains multiple CDMA forward-link channels. Similarly, digitized information from multiple mobile stations are transmitted to a base station over another 1.23 MHz RF band that contains multiple CDMA reverse-link channels.
More recently, third-generation broadband CDMA systems have been proposed in which forward-link spectrum is frequency subdivided into multiple RF bands. Within each of the RF bands, a different set of CDMA channels may be transmitted, where the CDMA channels in one RF band need not use mutually orthogonal coding in relation to another RF band. Third-generation CDMA cellular phone systems can implement an optional operation mode in which a high capacity data channel may be formed between a base station and a mobile station by using multiple CDMA channels, each of which is in a different one of the RF bands. This optional mode may be referred to as multi-carrier (MC) mode.
As shown in FIG. 2, in a multi-carrier CDMA (MC-CDMA) system, data may be transmitted from a base station to a mobile station using a 3.75 MHz of signal spectrum that is subdivided by frequency into different RF bands 211-213. Each of the RF bands 211-213 occupies its own RF spectrum and has a distinct carrier frequency, fc1, fc2, fc3. In proposed MC-CDMA standards, each RF band 211-213 occupies a 1.23 MHz spectrum and adjacent carrier frequencies fc1, fc2, fc3 of the RF bands have a minimum frequency spacing of 1.25 MHz. The carriers fc1, fc2, fc3 may be RF carriers in the cellular band (869-894 MHz) or in PCS band (1930-1990 MHz). Other carrier frequencies may also be used depending on available spectrum.
FIG. 3 shows a block diagram of a single-mode receiver that can be used in a single-mode MC-CDMA mobile station or base station to simultaneously receive and decade CDMA channels in different RF bands. The receiver 300 receives a radio frequency (RF) MC-CDMA signal (an “RF MC-CDMA” signal) from antenna 301 (which may be coupled to receiver 300 through duplexer 302) and provides decoded data to other circuitry in the mobile station or base station on outputs 331-333. For example, outputs 331-333 may be coupled to an audio decompression circuit or a data storage device.
The receiver 300 includes an initial amplification stage 303 to amplify the relatively weak RF MC-CDMA signal received by antenna 301. The amplification stage 303 may include low noise amplifiers (LNAs), band pass filters (BPFs) and radio frequency amplifiers (RFAs) that operate over a broad range of RF frequencies. An intermediate frequency MC-CDMA signal (an “IF MC-CDMA” signal) is then formed by down-converting the amplified RF MC-CDMA signal using a down-converter 304 (also known as a “signal mixer” or “mixer”). The down-converter 304 reduces the carrier frequencies fc1, fc2, and fc3, of received RF bands 211-213 (FIG. 2) by the frequency fLO of a local oscillator 305 signal. As a result, the carrier frequencies fc1, fc2, and fc3 of the RF bands 211-213 are reduced to intermediate frequency (IF) carriers fIF1, fIF2, and fIF3, respectively, where fIF1=fc1−fLO, fIF2=fc2−fLO, and fIF3=fc3−fLO. The resulting IF MC-CDMA signal is then coupled through a band pass filter 306 and a variable gain amplifier 307. The bandwidth of the band pass filter 306 is approximately equal to the bandwidth of the three IF MC-CDMA bands (e.g., approximately 3.7 MHz total).
An I/Q quadrature mixer stage 350 is then used to down-convert the amplified IF MC CDMA signal to produce an amplified and filtered base band signal from the IF MC CDMA signal. The I/Q quadrature mixer stage 350 down-converts the IF MC-CDMA signal to quadrature base band (BB) “I” and “Q” signals using down-converters 308 and 309. Down-converters 308 and 309 reduce the carrier frequencies of the IF MC-CDMA signal by the frequency fLO2 of another local oscillator 320. The local oscillator frequency fLO2 may be equal to the intermediate carrier frequency, fIF2, of the channel 112. Frequency folding occurs due to down-conversion using a local oscillator frequency FLO2=FIF2. As a result, each down-converter 308 and 309 will output a base band (BB) MC-CDMA signal having a bandwidth of approximately 1.875 MHz.
Frequency folding at down-converters 308 and 309 can result in the corruption of the information spectrum. Within the I/Q quadrature mixer stage 350, loss of information is prevented by forming and separating I and Q channel information. This can be accomplished by phase-shifting the local oscillator signal used for down-conversion at down-converter 308 by π/2 radians with respect to the oscillator signal used at 309. The resulting base band (BB) I and Q signals are then amplified, filtered, and recombined to reconstruct the data and thus contain the modulated spread CDMA data received in the three RF bands 211-213.
Amplification and filtering of baseband I and Q signals is performed using circuitry 310-313 and 314-317, respectively. Undesired high frequency components in the BB I signal from down-converter 308 are attenuated using a low pass filter 310. To minimize undesired signals, the low pass filter 310 should have a cutoff frequency approximately equal to the bandwidth of the BB I signal (e.g., 1.875 MHz). The filtered BB I signal is then amplified by amplifier 311 and digitally sampled at analog-to-digital converters (ADC) 312. ADC 312 forms a digital representation of the BB I signal. In general, the ADC 312 will sample the BB I signal at a rate of at least twice the BB I signal's highest frequency (resulting, in this example, in a minimum sampling rate of 3.75 MHz). The digital representation of the BB I signal may be filtered again by a digital low pass filter 313 with a 1.875 MHz bandwidth to further remove any residual high frequency interference signals or undesired signals introduced by amplification or analog-to-digital conversion. The BB Q signal may be similarly processed by circuit elements 314-317.
The digitized representations of the BB I and BB Q signals are then summed together using a complex number summing circuit 318. Summing the BB I and BB Q signals results in a full bandwidth (e.g., 3.75 MHz) unfolded signal that contains three base-band signals, each of which corresponds to one of the original three RF bands 211-213. The three base band signals are then separated from each other by a digital filter bank 319 and provided to different RAKE receiver fingers 321-323. The fingers 321-323 can each despread a CDMA channel from its corresponding base band signal to produce digital output 331-333, respectively.
In general, the bandpass filters and low pass filter used in the MC-CDMA receiver architecture 300 are limited to, or optimized for, performance in a single mode (e.g., MC-CDMA). It may, however, be desirable to build a mobile station or other receiving apparatus that can operate in multiple modes. For example, a receiver that can operate in both MC-CDMA mode and IS-95 may be desirable. Referring to FIGS. 3 and 4, to adapt the single-mode receiver 300 for use in multiple modes, filters 306, 310, 313, 314, 317 in receiver 300 may each be replaced with a pair of switched filters (406A, 406B), (410A, 410B), (413A, 413B), (414A, 414B), (417A, 417B), respectively. The first filter in each pair (the “A” filter) may be used to receive a 3.85 MHz MC-CDMA signal and may have a filter value equal to the corresponding filter in receiver 300. The second filter in each pair (the “B” filter) may have a filter value used for filtering a 1.23 MHz IS-95 RF band. For example, filter 406B may be a 1.23 MHz band-pass filter, and filters 410B, 414B, 413B, 417B may each be 0.65 MHz low pass filter. In addition, oscillators 405 and 420 are frequency-selectable to conform to the particular transmission frequencies used in each mode. The use of multiple filter sets to receive different types of CDMA signals may increase the complexity, cost, and/or power use of the receiver 400 or may have other disadvantages. Consequently, alternate receiver designs are desired. As third-generation systems are implemented, initial coverage may be limited. Consequently, a mobile station that can transmit and receive second-generation CDMA signals as well as third-generation CDMA signals is desirable. Multi-mode receivers may require additional filtering, switching, and amplification circuitry that is mode-specific to implement multiple modes. This additional circuitry may add additional size, cost, and complexity to the receiver. Consequently, it is desirable to have a flexible receiver design in which mode-specific circuitry is reduced.